The invention relates to glasses having a refractive index of .gtoreq. 1.60 and a linear thermal expansion coefficient of 45 - 58 .times. 10.sup.-.sup.7 /.degree. C, and especially to such glasses which can be fused with the known phototropic borosilicate glasses to form multifocal spectacle lenses wherein the near-portion glass is preferably also phototropic.
Phototropic borosilicate glasses are described in German Offenlegungsschrifts Nos. 1,924,493 and 2,223,629, the former relating to glasses with refractive indices nd .ltorsim. 1.53.
German Offenlegungsschrift No. 2,223,629 (corresponding to U.S. parent application Ser. No. 359,624) describes phototropic glasses which are suitable as the near-portion for fusion with phototropic far-portion glasses for multifocal spectacle lenses, and which have a linear thermal expansion coefficient between 45 and 58 .times. 10.sup.-.sup.7 /.degree. C and a refractive index nd higher than 1.60. These glasses are characterized by a composition range which, after synthesis, contains the following oxides in addition to phototropicity-imparting silver and halogens:
30 - 45 % by weight SiO.sub.2 + B.sub.2 O.sub.3 PA1 0 - 8 % by weight alkali oxides PA1 24 - 54 % by weight Al.sub.2 O.sub.3 + La.sub.2 O.sub.3 PA1 0 - 25 % by weight ZnO + ZrO.sub.2 + TiO.sub.2 PA1 2 - 25 % by weight ZnO + ZrO.sub.2 + TiO.sub.2 PA1 6 - 26 % by weight PbO PA1 0 - 8 % by weight alkaline earth oxide.
and also advantageously
One particularly suitable composition range for these phototropic near-portion glasses is, in percent by weight:
______________________________________ SiO.sub.2 5 - 30 % by weight B.sub.2 O.sub.3 7 - 35 % by weight PbO 6 - 26 % by weight ZnO 0 - 15 % by weight La.sub.2 O.sub.3 12 - 30 % by weight Al.sub.2 O.sub.3 12 - 25 % by weight ZrO.sub.2 0 - 6 % by weight TiO.sub.2 0 - 3 % by weight K.sub.2 O 0 - 2 % by weight Na.sub.2 O 0 - 2 % by weight Li.sub.2 O 0 - 4 % by weight Ag.sub.2 O 0.1 - 1.8 % by weight CuO 0 - 0.05 % by weight CoO 0 - 0.01 % by weight ______________________________________
and the following anion proportions in grams:
______________________________________ Cl 0.2 - 4.5 Br + I 0 - 4.0 F 0 - 4.0 ______________________________________
replacing the oxygen in grams, based on 100 g of glass oxide, with the following composition conditions being maintained:
______________________________________ sum of alkali oxides 0.2 - 8 sum of Al.sub.2 O.sub.3 + La.sub.2 O.sub.3 24.0 - 54 sum of alkaline earth oxides 0 - 8, in which MgO is less than 5.0 sum of ZnO + ZrO.sub.2 + TiO.sub.2 2 - 25 sum of the oxides of Bi, Ta, Nb, W 0 - 5. ______________________________________
Using these near-portion glass materials, it is possible by conventional melting methods to produce glasses which provide good multifocal lenses after having been fused with phototropic far-portion glasses.
Owing to their high content of lanthanum oxide, in combination with boron oxide, aluminum oxide, lead oxide, zirconium dioxide and titanium dioxide, the glasses according to Offenlegungsschrift 2,223,629 have, unfortunately, a tendency to crystallization, and in light of the temperature-viscosity properties of the glass, substantial difficulties are encountered in the continuous production of such bulk glasses in tanks.